Set scale of object by giving distance between two points
If you need to do this based on an off-axis dimension, or some other measurement like the area of a face, you can use the add-on "measureit" (included with the Blender 2.79 distribution), which can be found in the 3d view section of Add-Ons in the User Settings. 

When this is activated, a number showing the measurement you want appears on the 3dView and is updated in real time. You can select all the points of the object in Edit Mode and resize the object while watching this measurement change, then stop resizing when the measurement reaches the desired value.